The present invention relates to a method for calibrating equipment for detecting impurities in transparent material, said detecting equipment comprising a light source to illuminate the material, a camera to detect light transmitted through the material and signal-processing apparatus for processing and analysing signals from the camera representing the light transmission through the material.
The above-mentioned equipment for detecting impurities may be, for instance, of the type described in patent application SE 9901291-6 filed simultaneously.
As can be seen in FIG. 1 such an equipment comprises a light source 4 and a detector 6, between which an extruded polyethylene or polypropylene material 2 is caused to pass. A CCD linear camera is preferably used as detector. The camera 6 emits light transmission data for each individual scanned pixel. An xe2x80x9carea of interestxe2x80x9d is determined from these data in the material 2, in a first comparator 8 and these xe2x80x9careas of interestxe2x80x9d are stored in a buffer memory 10. A reference forming unit 16 is arranged to form a suitable reference value for the comparator 8, in order to determine said xe2x80x9careas of interestxe2x80x9d by comparison of the measured values with the reference value.
The buffer memory 10 is read by a digital signal processor 12. Several such processors 12, 12xe2x80x2, 12xe2x80x3 may possibly be arranged to operate in parallel if necessary. The light transmission data is analysed with the aid of the signal processor 12, to determine the type of impurity as well as its shape and extension. The result of this analysis is then transferred to a system computer 14 to be compiled and reported for classification of the material 2 under inspection, for instance.
The object of the present invention is to provide a method for reliable calibration of this type of detecting equipment, wherein the calibration method can be documented, enabling it to be traced backwards, as is required under the ISO 9000 standard.
This object is achieved with a method of the type described in the introduction having the characteristics defined in claim 1.
The method according to the invention offers a calibration method that is traceable in accordance with the requirements of ISO 9000, and no special adjustment members are needed for the relevant detecting equipment which a user might come into contact with, thereby unintentionally ruining the calibration. The calibration performed by the method according to the invention will thus remain over a period of time.
According to advantageous embodiments of the method according to the invention, said dark areas are produced with predetermined larger sizes on the calibration film, after which the areas are reduced to the desired sizes for the calibration. The spots and rings are reduced, suitably using a repro- or microfilm camera, to sizes in the range of 30-100 xcexcm. In principle any shape of patches and rings whatsoever can be used and these are made entirely black, i.e. completely untransparent for light, thus allowing the edge to be detected.
According to another embodiment of the method according to the invention, the dark areas are repeated on the calibration film with predetermined spacings so as to enable determination of the standard deviation in the calibration.
According to yet another advantageous embodiment of the method according to the invention, a scale is applied in the measuring position of the detecting equipment, with the aid of which the desired pixel resolution is adjusted and the camera focus fixed on the measuring position before the calibration film is scanned. The scale used for this adjustment is preferably one designed for this purpose, known as Heidenhain scale. After the pixel resolution has been adjusted, the setting of the camera is fixed mechanically with the camera lens focused on the measuring point and the camera adjustment can be maintained like that afterwards.
In order to explain the invention in more detail one embodiment of the method according to the invention will be described more fully by way of example with reference to FIG. 2.